Many of the ducks were resting with their bills tucked in. But the shape and position of the tail offers a helpful hint to their identity. This species is a member of the subfamily of ducks known as stiff-tailed ducks.
A pair of Ruddy Ducks (Oxyura jamaicensis).
Here's a close-up of the male once he became alert:
Note the solid white cheek patch, black cap, blue bill, and rust-colored sides.
For comparison, here's the female:
She has dark streak across her cheek, a brown cap, gray bill, and brown sides.
At this time of year, not all of the males have rusty sides and blue bills — some have yet to develop their full breeding plumage. But you can still identify them by their solid white cheeks.
In the picture below, how many males and females do you see?
Answer: There are three males and three females. From bottom to top: one male at bottom, then two females above him, then two males above them (rust-colored on left), and then one female at top.
In preparing for this post, I read in The Dictionary of American Bird Names that Ruddy Ducks used to be called "Jamaican Shovelers." Jamaica because that's where one of the first specimens was collected, and shoveler because of the way they use their broad bills to sift food from the water (similar to Northern Shovelers).
Most Ruddy Ducks will leave coastal locations by about mid-May to nest at inland ponds and lakes with marsh vegetation.